Schemas permission in SQL DB

I am a fresh admin in SQL DB and I need to learn how to establish the security settings in SQL DB. I have encountered an issue that is somehow unclear or even weird for me.

I know that if one user has Fixed Database-Level Roles db_datareader, he or she can perform "SELECT" on all user tables in DB.

But in my SQL DB, even the user which has already been added to that role can not perform "SELECT" on any table/view unless the "SELECT" permission is explicitly granted in "Schemas settings" which implies db_datareader role does not work at all.

Could anyone please give me advice on this? Many thanks